Kito, are you talking about using your own characters which you created for COH - your 'alts' or 'toons'? and just modifying them for your story setting?
IF that's the case, you can tweak your backstory, call the powers something different, and alter the general settings of the world you want in your writing. I believe that's what Mercedes Lackey did.
Also, the Invasion series she wrote incorporated, if I understand rightly, characters used by other people she played and RP'ed with in COH. I think for those instances, the writing was something of a collaborative effort with them, which is trickier, but possible.